Process for quenching hydrocarbon cracking apparatus effluent

ABSTRACT

Apparatus and process for compressing and quenching a cracked gas stream from a hydrocarbon cracking furnace including the step of feeding furnace output directly into an ejector in the effluent line, the ejector acting to quench and compress the effluent by injection of pressurized motive fluid into the ejector thereby rapidly mixing the motive fluid with the effluent for quick quenching and compression to prevent coke build-up and allow efficient heat exchanger and low pressure furnace operation.

This is a continuation of copending application Ser. No. 07/136,925(abandoned) filed on Oct. 19, 1987 and 07/274,623 (abandoned) filed onNov. 22, 1988.

BACKGROUND OF INVENTION

Most of the ethylene produced in the world is made via the steamcracking process. This process usually consists of a feedstock (such asethane, propane, butane, naphtha or gasoil) which is heated rapidly tohigh temperatures within tubular coils where the cracking reactionsoccur. The steam cracking furnace provides heat for the crackingreactions by burning fuel and transferring heat to the tubular coilswhich lie within the furnace firebox.

Steam is normally added to the feedstock in the coils prior to theradiant section of the furnace to provide the following benefits:

a) Reduce the hydrocarbon partial pressure within the coils to improveproduct yields.

b) Reduce coking rate within the coils.

c) Increase coil life by reducing carburization rate.

The steam cracking furnace is normally the key equipment item affectingprofitability within a petrochemical plant. As such, much work has beendone over the last 20 years to improve furnace performance; particularlyfeedstock flexibility, product yields and energy efficiency.

Product yields have been improved in recent years by reducing theresidence time of the feedstock and products within the radiant sectionof the furnace and in the furnace coil outlet piping upstream of thequench points or Transfer Line Exchanger (T.L.E.)--see U.S. Pat. No.3,923,921. At reduced residence times, coil average and coil outlettemperatures have increased to maintain feedstock conversion or crackingseverity. At higher coil outlet temperatures, the need to very rapidlyquench the cracking reactions becomes more important since this unfiredresidence time can result in rapid over-conversion of the feed and/orincreased tar and coke formation. Current practice in the petrochemicalindustry is to locate quench points or T.L.E.'s relatively close to thefurnace coil outlet and the hot furnace effluent is cooled/quenched to apoint where most cracking reactions stop within a period of 30 to 50milliseconds after exiting the furnace.

When the hot furnace effluent leaves the furnace, it can be quenchedwith an oil or water spray--see U.S. Pat. No. 4,599,478 and/or cooledusing a T.L.E. Normal practice is that an oil spray is used when thecracking feedstock is gasoil or heavier and a T.L.E. is used for lighterfeedstocks such as naphtha, L.P.G. and ethane. Using a T.L.E. is moreenergy efficient than oil quench since heat is recovered from thefurnace effluent at a higher temperature level. Oil quench is normallyemployed for heavy feedstocks because the large tar and coke yields fromthem rapidly foul downstream equipment such as T.L.E.'s--see, forexample, U.S. Pat. No. 4,444,697.

There are many T.L.E. designs and sometimes, in non-gasoil service, twoT.L.E.'s are placed in series to extract the maximum amount of highlevel heat from the process stream. The first T.L.E. in a series iscalled the primary T.L.E. and the main functions of this exchanger areto very rapidly cool the furnace effluent and generate high pressuresteam. The next T.L.E. is ca)led the secondary T.L.E. and its mainfunctions are to cool the furnace effluent to as low a temperature aspossible consistent with efficient primary fractionator or quench towerperformance and generate medium to low pressure steam.

The drive towards higher energy efficiency within petrochemical plantsin recent years has led to the development of T.L.E.'s that will copewith some gasoil feedstocks. These T.L.E.'s operate at highertemperatures than those in non-gasoil service and generate higherpressure steam to minimise the fouling caused by tar and cokedeposition.

The deposition of coke within the cracking coil and in the quench pointsor T.L.E.'s is a major operating problem with steam cracking furnaces.The coke build-up finally limits furnace throughput (via a coiltemperature constraint or unacceptably high pressure drops). The coke isremoved by burning it off the metal surfaces (in an operation calleddecoking).

A major problem with existing cracking furnaces is the high coil outletpressure that results from the pressure drop between the furnace coiloutlet and the inlet of the process gas compressor; as the gas flowsthrough piping, T.L.E.'s, fractionation and/or quench towers; and thesafety requirement to maintain a process gas compressor suction pressureabove atmospheric. Unfortunately this high pressure adversely affectsthe efficiency of the cracking reaction in the furnace. It has beenrecognised that a lowering of the pressure of the gas in the furnaceoutlet leads to improved product yields because there is a closecorrelation between the cracking reactions and the outlet gas pressure.

The present invention has as its principal object the provision of amotive fluid ejector, for lowering the furnace coil outlet pressure bycompressing the furnace effluent to sufficiently high pressures at theejector outlet to satisfy the pressure drop requirements of equipmentbetween the ejector and the inlet to a process gas compressor, and atthe same time to rapidly quench the temperature of the effluent gas onexiting the cracking furnace. A further objective is to control thequenching temperature so that the cracking reaction is stopped yetprovides adequately high temperature effluent for efficient heatexchanger operation and less energy loss.

The present invention provides for relatively low furnace oil outletpressures in the cracking furnace thus allowing relatively efficientcracking and therefore favourable product yields.

Accordingly, with the present invention, the amount of steam that isadded to the coils prior to the radiant section of a steam crackingfurnace may be significantly reduced with resultant energy savings.

SUMMARY OF INVENTION

There is provided according to the present invention a process andapparatus for quenching a cracked gas stream from a hydrocarbon crackingfurnace having a heating coil in the radiant section of the furnacewhere feedstock is heated and cracked, and an effluent line downstreamof the heating coil al the furnace outlet, wherein a venturi ispositioned in said effluent line as close as practicable to said furnaceoutlet, said venturi receiving furnace effluent and a motive fluid torapidly mix said fluid with said effluent to quench and compress saideffluent and motive fluid mixture.

Conveniently the invention includes the use of two ejectors in series toquench, cool and compress the effluent of a steam cracking furnace. Alsoit may be desirable to use a process computer to compute varioustemperatures, flow rates and pressures to optimise the performance ofthe two ejectors.

The novelty associated with the invention is the combination of: ejectorgeometry and design, position of ejector on the furnace outlet piping,the use of steam, water or oil as the ejector motive fluid and the useof an ejector as a compressor at the coil outlet to vary coil outletpressure to achieve the following desirable features:

1) Very low furnace coil outlet pressure (down to 1 p.s.i.g. from anormal of 10-15 p.s.i.g.).

2) Low unfired residence time above 1200° F. of the furnace effluent(down to 5 to 10 milliseconds).

3) Reduced hydrocarbon partial pressure during quenching as a resultof 1) above and the addition of steam within the ejector.

4) Reduced tar and coke formation and deposition within the pyrolysiscoil as a result of 1) above.

5) Suppression of the hydrocarbon dew point of the furnace effluent as aresult of 1), 2), 3) and 4) above.

6) Reduced fouling of downstream equipment such as quench points andT.L.E.'s due to 2) above resulting in less tar/coke formation outsidethe furnace, 4) and 5) above.

7) Improved product yields as a result of 1), 2), 3) and 4) above.

8) Increased run length of the pyrolysis coil due to 4) above.

9) Increased run length of quench points and/or T.L.E.'s due to 6)above.

10) For gasoil feedstocks, improved product separation within theprimary fractionator due to additional stripping steam from the ejectorsteam.

11) Allow higher process gas compressor suction pressure andconsequently reduced horsepower/high pressure steam requirement and/orprevent, or remove, bottlenecks in the process gas compressor andprimary fractionator or quench tower.

12) In heavy feedstock service, dew point suppression as a result of 5)above may allow installation of a T.L.E. immediately after the ejectorwith acceptable run lengths.

13) Reduction of steam injection volume into pyrolysis coil thusincreasing energy efficiency.

The two main functions of the ejector are to compress the furnaceeffluent and to rapidly mix and quickly quench the furnace effluent withmotive fluid. Thus the effluent has adequate pressure (typically 10-15p.s.i.g. and is in good condition to enter heat exchangers andfractionators.

The design of the ejector for commercial application can be madestandard for incorporation into new furnace quench/T.L.E. systems. Forretrofitting existing furnaces, custom designed ejectors may be usedtaking into account existing furnace/quench/T.L.E. geometry. Someprinciples that govern the final choice of steam nozzle geometry are asfollows:

1) Minimise coking within the ejector.

2) Maximise ejector efficiency.

3) Minimise erosion of the steam nozzle(s) and converging section duringnormal operation and during decokes.

4) A compromise of 1), 2) and 3) above may be forced byfurnace/quench/T.L.E. geometry.

DESCRIPTION OF DRAWINGS

FIGS. 1 and 2 are side views of two embodiments of ejector design.

FIGS. 3 and 4 are two embodiments of steam outlet nozzle design.

FIG. 5 shows a simple control system for flow of steam to the ejector.

FIG. 6 shows a further embodiment of a steam flow control system.

FIG. 7 is a schematic diagram of a further embodiment of an effluentquenching system.

DETAILED DESCRIPTION

Referring to FIG. 1, hot furnace effluent (1) leaves the furnace and assoon as practicable enters the ejector 20 which is of venturiconstruction receiving pressurised motive fluid such as steam, water oroil. The ejector may be welded to the furnace outlet line or flanged andbolted as shown (2). Medium pressure to high pressure steam (8) (100p.s.i.g. to 600 p.s.i.g.) is piped upstream of the convergent section ofthe ejector (4). Steam flows through a pipe (3) which is positioned inthe centreline of the ejector and then at sonic velocity through anozzle (9). The high velocity steam entrains furnace effluent and rapidmixing of steam and furnace effluent occurs in the convergent section(4), the mixing section (5) and in the divergent section (6). The rapidmixing results in rapid heat transfer and rapid cooling/quenching of thefurnace effluent. Pressure recovery occurs in the divergent section (6)and the gas mixture leaves the ejector (7). For high ejector efficiency,a divergent angle (10) of between 4° and 7° is desirable. Theconvergent/divergent nature of the ejector coupled with the highvelocity of the motive steam allows the ejector to act as a compressoron the furnace effluent. Thus the furnace may operate at lower thanconventional pressures because of the increase in pressure in theeffluent line created by the ejector.

FIG. 2 shows an ejector with a different steam nozzle design. Steam (8)enters a steam chest (3) which supplies steam to a nozzle arrangement(11).

FIGS. 3 and 4 show two options for the nozzle arrangement as viewed fromview A. In FIG. 3, between 4 and 50 holes (11) are spread evenly aroundthe circumference of the nozzle.

In FIG. 4, an annular space (11) provides the steam flowpath.

FIGS. 5 and 6 show two extremes of control of the motive fluid flow tothe ejector. A simple control scheme is shown in FIG. 5 and consists ofa single pressure controller 15 varying fluid flow through control valve15(a) to control furnace coil outlet pressure.

FIG. 6 shows a more sophisticated control scheme in which a processcomputer 16 has the following inputs:

1) Furnace coil outlet pressure PT.

2) Ejector fluid flow FT.

3) Product yield analysis via a transfer line analyser TLA.

4) Steam balance data.

5) Programmable equipment constraints, steam values and product values.

The computer can evaluate the optimum ejector motive fluid flow in realtime based on the cost of ejector motive fluid vs. product yield creditsand output to the motive fluid control valve.

A more sophisticated system allows the computer to add motive fluid fromdifferent sources or pressure levels depending on the cost/benefitanalysis for the various fluids.

Referring to FIG. 7, the primary ejector is located as close aspracticable to the outlet of furnace 30 to minimise unfired residencetime of the furnace effluent. The motive fluid 8 introduced into theprimary ejector 20 rapidly mixes with and quenches the hot furnaceeffluent thereby stopping most of the chemical reactions occurring inthe effluent stream and increases the pressure of the stream.

On leaving the primary ejector 20, the process stream may be cooled byone or more transfer line exchangers 12 (TLE's) which recover heat fromthe process stream usually by generation of medium to high pressuresteam 11. The decision to use a TLE, or the decision on how many TLE'sto use, will depend on furnace feedstock type and individual planteconomics.

On leaving the last TLE, the process stream enters a secondary ejector50 which cools the process stream to a set temperature for entry intothe primary fractionator or quench tower 40. The process gas compressor41 acts to compress the output of the fractionator or quench tower topressures of order of 400 p.s.i.g.

Preferably the primary ejector motive fluid 8 will be steam with theoption of some water addition for temperature control of the primaryejector outlet temperature. Conveniently the secondary ejector motivefluid will be quench oil 14 if a primary fractionator 40 is useddownstream of this ejector or quench water 15 if a quench tower 40 isused.

The main functions of the primary ejector are to:

1. Rapidly mix motive fluid with hot furnace effluent to quench andcompress the hot furnace effluent.

2. Reduce unfired residence time above 1200° F. of the furnace effluent.

3. Suppress the hydrocarbon dew point of the furnace effluent.

4. Reduce tar and coke formation within downstream equipment such asTLE's.

5. Improve furnace yields as a result of 1. and 2. above.

The main functions of the secondary ejector are to:

1. Cool the process stream to the correct primary fractionator/quenchtower inlet temperature.

2. Reduce the primary ejector motive fluid flow.

The main functions of the combination of primary and secondary ejectorsare to:

1. Compress the furnace effluent from furnace coil outlet to primaryfractionator/quench tower.

2. Allow optimisation of the flows of primary ejector motive fluid andsecondary ejector motive fluid.

3. Allow reduction of furnace coil outlet pressure to improve furnaceproduct yields.

A process computer may be used to control and optimise the primary andsecondary ejectors. Referring to FIG. 7, the computer inputs and outputscan include the following:

    ______________________________________                                        Item      Computer Inputs                                                     ______________________________________                                        P1        Furnace coil outlet pressure.                                       T1        Furnace coil outlet temperature.                                    F1        Primary ejector motive fluid flow.                                  T2        Primary ejector motive fluid temperature.                           T3        Primary ejector outlet temperature.                                 P2        Primary ejector outlet pressure.                                    A         Product yield analysis via transfer line                                      analyser.                                                           P3        Secondary ejector inlet pressure.                                   T4        Secondary ejector inlet temperature.                                F2        High pressure generated steam flow.                                 F3        Secondary ejector motive fluid flow.                                P4        Secondary ejector motive fluid pressure.                            T5        Secondary ejector outlet temperature.                               P5        Secondary ejector outlet pressure.                                  P6        Process gas compressor suction pressure.                            T6        Process gas compressor suction                                                temperature.                                                        FF        Furnace feed flow rate.                                             ______________________________________                                    

Other factors include equipment constraints, steam balance data, andfeedstock and motive fluid costs; product and byproduct values;furnace/TLE run length, capacity and service factor credits.

The computer outputs may control the following parameters:

(i) Furnace feed flow.

(ii) Selection of primary ejector motive fluid source.

(iii) Secondary ejector motive fluid flow.

(iv) Secondary ejector motive fluid temperature (via motive fluid coolerbypassing).

(v) Process gas compressor suction pressure.

What is claimed is:
 1. A process for quenching the cracked gas effluentof a hydrocarbon cracking furnace having an effluent outlet, saidprocess comprising:(a) introducing said effluent through said outletinto one end of a compression zone external of said furnace andsufficiently close to said outlet as to provide minimum unfiredresidence time of said effluent between said outlet and said one end ofsaid compression zone; and (b) introducing a temperature quenching fluidinto said one end of said compression zone via an inlet closely adjacentsaid outlet and between said compression zone and said outlet and at anelevated velocity sufficient to entrain and quench said effluent andincrease the pressure of said effluent in said compression zonedownstream from said outlet, thereby reducing the pressure of saideffluent upstream from said outlet and reducing the hydrocarbonresidence time in said furnace.
 2. The process of claim 1 wherein saidcompression zone is in the form of a conduit converging in the directionof effluent flow.
 3. The process of claim 1 wherein said effluent andquenching fluid are introduced into and mixed in said compression zoneadjacent its inlet, the mixture of said effluent and fluid flowing fromsaid compression zone into a mixing zone and thereafter into a pressurerecovery zone.
 4. The process of claim 3 including further quenchingsaid mixture in said mixing zone.
 5. The process of claim 1 wherein saidquenching fluid comprises steam at a pressure of between 100 and 600psig and in sufficient amount to reduce the temperature of said effluentto less than 1200° F.